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Combustion properties of density separated inertinite macerals in the Herrin #6 and Murphysboro coal seamsYork, Christopher Daniel 01 January 2009 (has links)
The inertinite combustion reactivity of the Herrin #6 and the Murphysboro coal seams were examined. Three samples of the Herrin #6 seam were analyzed: A whole coal sample, a resonance disintegration milled sample, and a fusain lithotype sample. A fusain lithotype sample was also analyzed for the Murphysboro coal seam. Density gradient centrifugation (DGC) techniques were used to separate the inertinite components of each sample into 26 discrete density fractions. Fractions were chosen based upon changes in the density profiles. The thermal properties of the fractions were analyzed by thermogravimetric (TGA) analysis. The results showed that although the reactivity profiles for the coal samples were different, discontinuities occurred in all of the coal samples across the same density intervals. It also illustrated that the change between semifusinite and fusinite was not a continuous change as suggested by the definition (ICCP, 2001, but was instead shown to be discontinuous. Changes within the chemical structure were inferred from these discontinuities indicating differences within the semifusinite and fusinite maceral groups. The boundary between more/less reactive macerals was shown to increase with density as temperature increased indicating that at higher energies more macerals were reactive during the combustion process.
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The influence of potassium carbonate and potassium chloride during heat treatment of an inertinite-rich bituminous char / Kelebogile Ancient LeeuwLeeuw, Kelebogile Ancient January 2012 (has links)
Thermogravimetry, coupled with a mass spectrometer (TG-MS) was used to investigate
the catalytic effect potassium carbonate (K2CO3) and potassium chloride (KCl), on the char conversion and the product gas composition of chars derived from a South African inertinite-rich bituminous coal. Sequential leaching of the coal with HCl-HF-HCl was performed to reduce the mineral matter present in the coal. This was done in order to reduce possible undesirable interactions between the minerals and inorganic compounds in the coal during heat treatments. The leaching process substantially reduced the ash content from 21.5% to less than 3%.
K2CO3 and KCl [0.5, 1, 3, 5 K-wt %] were loaded to the demineralized coal, raw coal and
demineralized coal with added mineral mixture prior to charring. The mineral mixture was made up of kaolinite, quartz, pyrite, siderite, calcite, anastase and hydromagnesite. The ‘doped’ coal samples were then subjected to heat treatments in a CO2 atmosphere up to 1200 °C. The results obtained showed that both K2CO3 and KCl exhibit a catalytic effect on the char conversion during heat treatments in CO2 atmosphere and the char conversion was increased with increasing loadings up to 5 K-wt% of K2CO3 and KCl. The temperature ranges at which conversion occurred were found to be lower for K2CO3 than for KCl. Subsequently, char conversion occurred over a relatively narrower temperature range for K2CO3 than observed for KCl. The catalytic behaviour of K2CO3 and KCl was confirmed by the results obtained. The results also indicated that the catalytic influence of K2CO3 is greater than that of KCl and that KCl is more susceptible to deactivation by minerals and inorganic compounds present in the coal than K2CO3.
Different analytical techniques (XRF and XRD) were used to determine the extent of
interaction of the catalysts used with the char material in the 5 K-wt% ‘doped’ coal samples. From the XRF results, it was observed that the K2O content was reduced after heat treatments in CO2, however, no potassium crystalline phases were observed in the XRD results after heat treatments in CO2. The reduced K2O content may be attributed to the potassium been taken up in other mineral matter during char reaction with CO2, forming new amorphous inorganic complex compounds. Thus the potassium retained in the sample after heat treatment, indicated by the XRF results, may be in an amorphous phase.
Mass spectrometry (MS) indicated that temperatures at which the maximum rate of
evolution of gaseous species occurred were relatively lower for K2CO3 loaded char samples iv than observed for KCl loaded samples. In addition, no mass-to-charge ratio (m/z) peak at 39 atomic mass unit (amu) from the MS results was observed, indicating that no potassium was detected in the gaseous phases for all the char samples. The undetected potassium in the gaseous phase may be due to the detection limit of the MS equipment.
The MS results also indicated that addition of the catalyst facilitates the evolution of H2
from the coal char samples. Addition of the catalysts to the samples lowered the temperature at which maximum H2 was given off. The shift to lower temperatures was observed with increased catalyst loadings for both K2CO3 and KCl loaded samples. / Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 201
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The influence of potassium carbonate and potassium chloride during heat treatment of an inertinite-rich bituminous char / Kelebogile Ancient LeeuwLeeuw, Kelebogile Ancient January 2012 (has links)
Thermogravimetry, coupled with a mass spectrometer (TG-MS) was used to investigate
the catalytic effect potassium carbonate (K2CO3) and potassium chloride (KCl), on the char conversion and the product gas composition of chars derived from a South African inertinite-rich bituminous coal. Sequential leaching of the coal with HCl-HF-HCl was performed to reduce the mineral matter present in the coal. This was done in order to reduce possible undesirable interactions between the minerals and inorganic compounds in the coal during heat treatments. The leaching process substantially reduced the ash content from 21.5% to less than 3%.
K2CO3 and KCl [0.5, 1, 3, 5 K-wt %] were loaded to the demineralized coal, raw coal and
demineralized coal with added mineral mixture prior to charring. The mineral mixture was made up of kaolinite, quartz, pyrite, siderite, calcite, anastase and hydromagnesite. The ‘doped’ coal samples were then subjected to heat treatments in a CO2 atmosphere up to 1200 °C. The results obtained showed that both K2CO3 and KCl exhibit a catalytic effect on the char conversion during heat treatments in CO2 atmosphere and the char conversion was increased with increasing loadings up to 5 K-wt% of K2CO3 and KCl. The temperature ranges at which conversion occurred were found to be lower for K2CO3 than for KCl. Subsequently, char conversion occurred over a relatively narrower temperature range for K2CO3 than observed for KCl. The catalytic behaviour of K2CO3 and KCl was confirmed by the results obtained. The results also indicated that the catalytic influence of K2CO3 is greater than that of KCl and that KCl is more susceptible to deactivation by minerals and inorganic compounds present in the coal than K2CO3.
Different analytical techniques (XRF and XRD) were used to determine the extent of
interaction of the catalysts used with the char material in the 5 K-wt% ‘doped’ coal samples. From the XRF results, it was observed that the K2O content was reduced after heat treatments in CO2, however, no potassium crystalline phases were observed in the XRD results after heat treatments in CO2. The reduced K2O content may be attributed to the potassium been taken up in other mineral matter during char reaction with CO2, forming new amorphous inorganic complex compounds. Thus the potassium retained in the sample after heat treatment, indicated by the XRF results, may be in an amorphous phase.
Mass spectrometry (MS) indicated that temperatures at which the maximum rate of
evolution of gaseous species occurred were relatively lower for K2CO3 loaded char samples iv than observed for KCl loaded samples. In addition, no mass-to-charge ratio (m/z) peak at 39 atomic mass unit (amu) from the MS results was observed, indicating that no potassium was detected in the gaseous phases for all the char samples. The undetected potassium in the gaseous phase may be due to the detection limit of the MS equipment.
The MS results also indicated that addition of the catalyst facilitates the evolution of H2
from the coal char samples. Addition of the catalysts to the samples lowered the temperature at which maximum H2 was given off. The shift to lower temperatures was observed with increased catalyst loadings for both K2CO3 and KCl loaded samples. / Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 201
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A study of the influence of thermal drying on physical coal properties / M.J.G. Badenhorst.Badenhorst, Mathys Johannes Gerhardus January 2009 (has links)
One of the major issues facing the coal industry today is the decline in economically
recoverable reserves, especially in the Witbank 1 Mpumalanga region of South Africa. This necessitates a critical review of alternate coal sources. One such source was identified as previously discarded and currently arising coal fines. It is well known that great value lies within
these fines, but that the high moisture content associated with fine coal leads to thermal inefficiencies, handling problems and increased transport cost. This study will investigate thermal drying as a feasible solution to effectively remove this moisture. During thermal drying coal is fed into a high temperature environment; this can influence the physical and mechanical properties of the coal. The effects include swelling, caking, cracking, loss of water, loss of volatiles, and many more. These effects are investigated by means of
thennogravimetric analysis, scanning electron microscopy with a heating stage, proximate analysis and mercury intrusion. Coal samples with an average particle size of 500 um were selected for this study. It was found that: The rate of moisture loss up to temperatures between 150 and 200°C is at a maximum where after the rate declines up to temperatures between 350 and 450°C when primary devolatilisation initiates.
No visual changes in the coal are observed up to temperatures between 350 and 450°C.
A limited amount of volatiles evolve at a constant rate up to 250°C; this is not significant enough to decrease the calorific value of the coal.
Porosity changes in the coal are observed from temperatures as low as 250°C.
Thermal drying was found to be a feasible alternative to currently employed drying methods with 150°C selected as the optimal drying temperature. A thermal drying plant is proposed with recommendations for future work needed to realise such a plant. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2010
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A study of the influence of thermal drying on physical coal properties / M.J.G. Badenhorst.Badenhorst, Mathys Johannes Gerhardus January 2009 (has links)
One of the major issues facing the coal industry today is the decline in economically
recoverable reserves, especially in the Witbank 1 Mpumalanga region of South Africa. This necessitates a critical review of alternate coal sources. One such source was identified as previously discarded and currently arising coal fines. It is well known that great value lies within
these fines, but that the high moisture content associated with fine coal leads to thermal inefficiencies, handling problems and increased transport cost. This study will investigate thermal drying as a feasible solution to effectively remove this moisture. During thermal drying coal is fed into a high temperature environment; this can influence the physical and mechanical properties of the coal. The effects include swelling, caking, cracking, loss of water, loss of volatiles, and many more. These effects are investigated by means of
thennogravimetric analysis, scanning electron microscopy with a heating stage, proximate analysis and mercury intrusion. Coal samples with an average particle size of 500 um were selected for this study. It was found that: The rate of moisture loss up to temperatures between 150 and 200°C is at a maximum where after the rate declines up to temperatures between 350 and 450°C when primary devolatilisation initiates.
No visual changes in the coal are observed up to temperatures between 350 and 450°C.
A limited amount of volatiles evolve at a constant rate up to 250°C; this is not significant enough to decrease the calorific value of the coal.
Porosity changes in the coal are observed from temperatures as low as 250°C.
Thermal drying was found to be a feasible alternative to currently employed drying methods with 150°C selected as the optimal drying temperature. A thermal drying plant is proposed with recommendations for future work needed to realise such a plant. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2010
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Gasification kinetics of blends of waste tyre and typical South African coals / Chaitamwari GuraiGurai, Chaitamwari January 2015 (has links)
With increasing energy demand globally and, in particular, in South Africa coupled with
depletion of the earth’s fossil energy resources and growing problem of disposal of nonbiodegradable
waste such as waste tyres, there is a need and effort globally to find alternative
energy from waste material including waste tyres. One possible way of exploiting waste tyre
for energy or chemicals recovery is through gasification for the production of syngas, and this
is what was investigated in this study. The possibility of gasification of waste tyre blended with
coal after pyrolysis was investigated and two Bituminous coals were selected for blending with
the waste tyre in co-gasification. A sample of ground waste tyre / waste tire, WT, a high vitrinite
coal from the Waterberg coalfield (GG coal) and a high inertinite coal from the Highveld
coalfield (SF coal) were used in this investigation.
The waste tyre sample had the highest volatile matter content of 63.8%, followed by GG coal
with 27% and SF coal with 23.8%. SF coal had the highest ash content of 21.6%, GG coal had
12.6% and waste tyre had the lowest of 6.6%. For the chars, SF char still had the highest ash
of 24.8%, but WT char had higher ash, 14.7%, when compared to GG char with 13.9% ash.
The vitrinite content in GG coal was 86.3%, whilst in SF coal it was 25% and SF coal had a
higher inertinite content of 71% when compared to GG coal with 7.7%. SF char had the highest
BET surface area of 126m2/g, followed by GG char with 113m2/g, and WT had the lowest
value of 35.09m2/g. The alkali indices of the SF, WT and GG chars were calculated to be 8.2,
4.2 and 1.7 respectively.
Coal samples were prepared by crushing and milling to particle sizes less than 75μm before
charring in a packed bed balance reactor at temperatures up to 1000oC.Waste tyre samples were
charred at the same conditions before milling to < 75μm particle size. Coal and WT chars were
blended in ratios of 75:25, 50:50 and 25:75 before gasification experimentation. Carbon
dioxide gasification was conducted on the blends and the pure coal and WT chars in a
Thermogravimetric analyser (TGA) at 900oC, 925oC, 950oC and 975oC and ambient pressure.
100% CO2 was used at a flow rate of 2L/min.
Reactivity of the pure char samples was found to be in the order SF > GG > WT, and the
relationship between the coal chars’ reactivities could be explained by the high ash content of
the SF char and low reactivity of the WT char corresponds to its low BET surface area. In
general, the coal/WT char mixtures were less reactive than the respective coal, but more
reactive than the pure WT char, the only exception being the 75% GG char blend which was
initially more reactive than the GG char, and reactivity decreased with increasing WT content.
For all samples reactivity increased with increasing temperature.
The relationship between the reactivities of the GG char and its blends and that of the SF char
and its blends was found to be affected by the amount of WT char added, especially at the
lower temperatures 900oC and 925oC. SF coal is more reactive than GG coal, but at 900oC and
925oC, the reactivity of GG/WT blends improves in relation to the SF/WT blends with an
increase in the ratio of WT in the blends, i.e. the 25% GG char blend is more reactive than the
25% SF char blend. The reactivity of the coal/WT blends was also checked against predicted
conversion rates based on the conversion rates of the pure WT and coal samples. At 900oC and
925oC, the reactivities of the blends of both coal chars with WT char were found to be greater
than the predicted conversion rates, and for the GG/WT blends the deviation increased with
increasing WT ratios, while for the SF/WT blends the deviation increased with increasing SF
ratios. These findings suggest the presence of synergism or enhancement between the coal
chars and WT char in gasification reactions.
The random pore model (RPM) was used to model the gasification results and it was found to
adequately describe the experimental data. Activation energies determined with the RPM were
found to be 205.4kJ/mol, 189.9kJ/mol and 173.9kJ/mol for SF char, WT char and GG char
respectively. The activation energies of the coal/WT blends were found to be lower than those
of both the pure coal and the pure WT chars. For the GG/WT blends the activation energy
decreased with increasing WT char ratio, while for the SF/WT blends the activation energy
decreased with increasing SF char ratio.
The trends of the activation energies and conversion rates of the blends point to synergism or
enhancement between the coal and WT chars in CO2 gasification reactions, and in the GG/WT
blends this enhancement is driven more by the WT char, while in SF/WT blends it is driven by
SF chars. It is possible that enhancement of the reactions is caused by mineral matter catalysis
of the gasification reactions. The ash contents and alkali indices of the pure samples follow the
order SF > WT > GG. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
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Gasification and combustion kinetics of typical South African coal chars / Mpho RambudaRambuda, Mpho January 2015 (has links)
An investigation was undertaken to compare the kinetics of combustion and gasification
reactions of chars prepared from two South African coals in different reaction atmospheres:
air, steam, and carbon dioxide. The two original coals were characterised as vitrinite-rich
(Greenside) and inertinite-rich (Inyanda) coals with relatively low ash content (12.5-16.7 wt.
%, adb). Chars were prepared from the parent coals under nitrogen atmosphere at 900 °C.
Characterisation results show that the volatiles and moisture were almost completely driven
off from the parent coals, indicating that the pyrolysis process was efficient. Physicalstructural
properties such as porosity and surface area generally increased from the parent
coals to the subsequent chars. The heterogeneous char-gas reactions were conducted
isothermally in a TGA on ~1 mm size particles. To ensure that the reactions are under
chemical reaction kinetic control regime, different temperatures zones were selected for the
three different reaction atmospheres. Combustion reactivity experiments were carried out
with air in the temperature range of 387 °C to 425 °C; gasification reactivity with pure steam
were conducted at higher temperatures (775 °C - 850 °C) and within 825 °C to 900 °C with
carbon dioxide. Experimental results show differences in the specific reaction rate with
carbon conversion in different reaction atmospheres and char types. Reaction rates in all three
reaction atmospheres were strongly dependent on temperature, and follow the Arrhenius type
kinetics. All the investigated reactions (combustion with air and gasification with CO2 and
steam) were found to be under chemical reaction control regime (Regime I) for both chars.
The inertinite-rich coals exhibit longer burn-out time than chars produced from vitrinite-rich
coals, as higher specific reaction rate were observed for the vitrinite-rich coals in the three
different reaction atmospheres. The determined random pore model (RPM) structural
parameters did not show any significant difference during steam gasification of Greenside
and Inyanda chars, whereas higher structural parameter values were observed for Greenside
chars during air combustion and CO2 gasification (ψ > 2). However a negative ψ value was
determined during CO2 gasification and air combustion of Inyanda chars. The RPM
predictions was validated with the experimental data and exhibited adequate fitting to the
specific rate of reaction versus carbon conversion plots of the char samples at the different
reaction conditions chosen for this study. The activation energy determined was minimal for
air and maximum for CO2 for both coals; and ranged from 127-175 kJ·mol-1 for combustion,
214-228 kJ·mol-1 and 210-240 kJ·mol-1 for steam and CO2 gasification respectively. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
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Gasification kinetics of blends of waste tyre and typical South African coals / Chaitamwari GuraiGurai, Chaitamwari January 2015 (has links)
With increasing energy demand globally and, in particular, in South Africa coupled with
depletion of the earth’s fossil energy resources and growing problem of disposal of nonbiodegradable
waste such as waste tyres, there is a need and effort globally to find alternative
energy from waste material including waste tyres. One possible way of exploiting waste tyre
for energy or chemicals recovery is through gasification for the production of syngas, and this
is what was investigated in this study. The possibility of gasification of waste tyre blended with
coal after pyrolysis was investigated and two Bituminous coals were selected for blending with
the waste tyre in co-gasification. A sample of ground waste tyre / waste tire, WT, a high vitrinite
coal from the Waterberg coalfield (GG coal) and a high inertinite coal from the Highveld
coalfield (SF coal) were used in this investigation.
The waste tyre sample had the highest volatile matter content of 63.8%, followed by GG coal
with 27% and SF coal with 23.8%. SF coal had the highest ash content of 21.6%, GG coal had
12.6% and waste tyre had the lowest of 6.6%. For the chars, SF char still had the highest ash
of 24.8%, but WT char had higher ash, 14.7%, when compared to GG char with 13.9% ash.
The vitrinite content in GG coal was 86.3%, whilst in SF coal it was 25% and SF coal had a
higher inertinite content of 71% when compared to GG coal with 7.7%. SF char had the highest
BET surface area of 126m2/g, followed by GG char with 113m2/g, and WT had the lowest
value of 35.09m2/g. The alkali indices of the SF, WT and GG chars were calculated to be 8.2,
4.2 and 1.7 respectively.
Coal samples were prepared by crushing and milling to particle sizes less than 75μm before
charring in a packed bed balance reactor at temperatures up to 1000oC.Waste tyre samples were
charred at the same conditions before milling to < 75μm particle size. Coal and WT chars were
blended in ratios of 75:25, 50:50 and 25:75 before gasification experimentation. Carbon
dioxide gasification was conducted on the blends and the pure coal and WT chars in a
Thermogravimetric analyser (TGA) at 900oC, 925oC, 950oC and 975oC and ambient pressure.
100% CO2 was used at a flow rate of 2L/min.
Reactivity of the pure char samples was found to be in the order SF > GG > WT, and the
relationship between the coal chars’ reactivities could be explained by the high ash content of
the SF char and low reactivity of the WT char corresponds to its low BET surface area. In
general, the coal/WT char mixtures were less reactive than the respective coal, but more
reactive than the pure WT char, the only exception being the 75% GG char blend which was
initially more reactive than the GG char, and reactivity decreased with increasing WT content.
For all samples reactivity increased with increasing temperature.
The relationship between the reactivities of the GG char and its blends and that of the SF char
and its blends was found to be affected by the amount of WT char added, especially at the
lower temperatures 900oC and 925oC. SF coal is more reactive than GG coal, but at 900oC and
925oC, the reactivity of GG/WT blends improves in relation to the SF/WT blends with an
increase in the ratio of WT in the blends, i.e. the 25% GG char blend is more reactive than the
25% SF char blend. The reactivity of the coal/WT blends was also checked against predicted
conversion rates based on the conversion rates of the pure WT and coal samples. At 900oC and
925oC, the reactivities of the blends of both coal chars with WT char were found to be greater
than the predicted conversion rates, and for the GG/WT blends the deviation increased with
increasing WT ratios, while for the SF/WT blends the deviation increased with increasing SF
ratios. These findings suggest the presence of synergism or enhancement between the coal
chars and WT char in gasification reactions.
The random pore model (RPM) was used to model the gasification results and it was found to
adequately describe the experimental data. Activation energies determined with the RPM were
found to be 205.4kJ/mol, 189.9kJ/mol and 173.9kJ/mol for SF char, WT char and GG char
respectively. The activation energies of the coal/WT blends were found to be lower than those
of both the pure coal and the pure WT chars. For the GG/WT blends the activation energy
decreased with increasing WT char ratio, while for the SF/WT blends the activation energy
decreased with increasing SF char ratio.
The trends of the activation energies and conversion rates of the blends point to synergism or
enhancement between the coal and WT chars in CO2 gasification reactions, and in the GG/WT
blends this enhancement is driven more by the WT char, while in SF/WT blends it is driven by
SF chars. It is possible that enhancement of the reactions is caused by mineral matter catalysis
of the gasification reactions. The ash contents and alkali indices of the pure samples follow the
order SF > WT > GG. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
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Gasification and combustion kinetics of typical South African coal chars / Mpho RambudaRambuda, Mpho January 2015 (has links)
An investigation was undertaken to compare the kinetics of combustion and gasification
reactions of chars prepared from two South African coals in different reaction atmospheres:
air, steam, and carbon dioxide. The two original coals were characterised as vitrinite-rich
(Greenside) and inertinite-rich (Inyanda) coals with relatively low ash content (12.5-16.7 wt.
%, adb). Chars were prepared from the parent coals under nitrogen atmosphere at 900 °C.
Characterisation results show that the volatiles and moisture were almost completely driven
off from the parent coals, indicating that the pyrolysis process was efficient. Physicalstructural
properties such as porosity and surface area generally increased from the parent
coals to the subsequent chars. The heterogeneous char-gas reactions were conducted
isothermally in a TGA on ~1 mm size particles. To ensure that the reactions are under
chemical reaction kinetic control regime, different temperatures zones were selected for the
three different reaction atmospheres. Combustion reactivity experiments were carried out
with air in the temperature range of 387 °C to 425 °C; gasification reactivity with pure steam
were conducted at higher temperatures (775 °C - 850 °C) and within 825 °C to 900 °C with
carbon dioxide. Experimental results show differences in the specific reaction rate with
carbon conversion in different reaction atmospheres and char types. Reaction rates in all three
reaction atmospheres were strongly dependent on temperature, and follow the Arrhenius type
kinetics. All the investigated reactions (combustion with air and gasification with CO2 and
steam) were found to be under chemical reaction control regime (Regime I) for both chars.
The inertinite-rich coals exhibit longer burn-out time than chars produced from vitrinite-rich
coals, as higher specific reaction rate were observed for the vitrinite-rich coals in the three
different reaction atmospheres. The determined random pore model (RPM) structural
parameters did not show any significant difference during steam gasification of Greenside
and Inyanda chars, whereas higher structural parameter values were observed for Greenside
chars during air combustion and CO2 gasification (ψ > 2). However a negative ψ value was
determined during CO2 gasification and air combustion of Inyanda chars. The RPM
predictions was validated with the experimental data and exhibited adequate fitting to the
specific rate of reaction versus carbon conversion plots of the char samples at the different
reaction conditions chosen for this study. The activation energy determined was minimal for
air and maximum for CO2 for both coals; and ranged from 127-175 kJ·mol-1 for combustion,
214-228 kJ·mol-1 and 210-240 kJ·mol-1 for steam and CO2 gasification respectively. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
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The molecular structure of selected South African coal-chars to elucidate fundamental principles of coal gasification / Mokone Joseph RobertsRoberts, Mokone Joseph January 2015 (has links)
Advances in the knowledge of chemical structure of coal and development of high
performance computational techniques led to more than hundred and thirty four
proposed molecular level representations (models) of coal between 1942 and 2010.
These models were virtually on the carboniferous coals from the northern hemisphere.
There are only two molecular models based on the inertinite- and vitrinite-rich coals
from the southern hemisphere. The current investigation is based on the chars derived
from the Permian-aged coals in two major South African coalfields, Witbank #4 seam
and Waterberg Upper Ecca. The two coals were upgraded to 85 and 93% inertinite- and
vitrinite-rich concentrates, on visible mineral matter free basis. The coals were slow
heated in inert atmosphere at 20 ℃ min-1 to 450, 700 and 1000 ℃ and held at that
temperature for an hour. After the HCl-HF treatment technique at ambient temperatures,
the characteristics of the coals and chars were examined with proximate, ultimate,
helium density, porosity, surface area, petrographic, solid-state 13C NMR, XRD and
HRTEM analytical techniques. The results largely showed that substantial transitions
occurred at 700-1000 ℃, where the chars became physically different but chemically
similar. Consequently, the chars at the highest temperature (1000 ℃) drew attention to
the detailed study of the atomistic properties that may give rise to different reactivity
behaviours with CO2 gas.
The H/C atomic ratios for the inertinite- and vitrinite-rich chars were respectively 0.31
and 0.49 at 450 ℃ and 0.10 and 0.12 at 1000 ℃. The true density was respectively 1.48
and 1.38 g.cm-3 at 450 ℃ and 1.87 and 1.81 g.cm-3 at 1000 ℃. The char form results
from the petrographic analysis technique indicated that the 700-1000 ℃ inertinite-rich
chars have lower proportions of thick-walled isotropic coke derived from pure vitrinites
(5-8%) compared with the vitrinite-rich chars (91-95%). This property leads to the
creation of pores and increases of volume and surface area as the softening walls
expand. It was found that the average crystallite diameter, La, and the mean length of
the aromatic carbon fringes from the XRD and HRTEM techniques, respectively, were in
good agreement and made a definite distinction between the 1000 ℃ inertinite- and vitrinite-rich chars. The crystallite diameter on peak (10) approximations, La(10), of
37.6Å for the 1000 ℃ inertinite-rich chars fell within the HRTEM’s range of minimummaximum
length boundary of 11x11 aromatic fringes (27-45Å). The La (10) of 30.7Å for
the vitrinite-rich chars fell nearly on the minimum-maximum length range of 7x7
aromatic fringes (17-28Å.) The HRTEM results showed that the 1000 ℃ inertinite-rich
chars comprised a higher distribution of larger aromatic fringes (11x11 parallelogram
catenations) compared with a higher distribution of smaller aromatic fringes (7x7
parallelogram catenations).
The mechanism for the similarity between the 700-1000 ℃ inertinite- and vitrinite-rich
chars was the greater transition occurring in the vitrinite-rich coal to match the more
resistant inertinite-rich coal. This emphasised that the transitions in the properties of
vitrinite-rich coals were more thermally accelerated than those of the inertinite-rich
coals. The similarity between the inertinite- and vitrinite-rich chars was shown by the
total maceral reflectance, proximate, ultimate, skeletal density and aromaticity results.
Evidence for this was the carbon content by mass for the inertinite- and vitrinite-rich
chars of respectively 90.5 and 85.3% at 450 ℃ and 95.9 and 94.1% at 1000 ℃. The
aromaticity from the XRD technique was respectively 87 and 77% at 450 ℃ and 98 and
96% at 1000 ℃. A similar pattern was found in the hydrogen and oxygen contents, the
atomic O/C ratios and the aromaticity from the NMR technique.
The subsequent construction of large-scale molecular structures for the 1000 ℃
inertinite-rich chars comprised 106 molecules constructed from a total of 42929 atoms,
while the vitrinite-rich char model was made up of 185 molecules consisting of a total of
44315 atoms. The difference between the number of molecules was due to the
inertinite-rich char model comprising a higher distribution of larger molecules compared
with the vitrinite-rich char model, in agreement with the XRD and HRTEM results. These
char structures were used to examine the behaviour on the basis of gasification
reactivity with CO2.
The density functional theory (DFT) was used to evaluate the interactions between CO2
and the atomistic representations of coal char derived from the inertinite- and vitrinite rich South African coals. The construction of char models used the modal aromatic
fringes (fringes of highest frequencies in size distributions) from the HRTEM, for the
inertinite- and vitrinite-rich chars, respectively (11x11 and 7x7 parallelogram-shaped
aromatic carbon rings). The structures were DFT geometrically optimized and used to
measure reactivity with the Fukui function, f+(r) and to depict a representative reactive
carbon edge for the simulations of coal gasification reaction mechanism with CO2 gas.
The f+(r) reactivity indices of the reactive edge follows the sequence: zigzag C remote
from the tip C (Czi = 0.266) > first armchair C (Cr1 = 0.087) > tip C (Ct = 0.075) > second
armchair C (Cr2 = 0.029) > zigzag C proximate to the tip C (Cz = 0.027). The DFT
simulated mean activation energy, ΔEb, for the gasification reaction mechanism
(formation of second CO gas molecule) was 233 kJ mol-1. The reaction for the formation
of second CO molecule is defines gasification in essence. The experimental activation
energy determined with the TGA and random pore model to account essentially for the
pore variation in addition to the gasification chemical reaction were found to be very
similar: 191 ± 25 kJ mol-1 and 210 ± 8 kJ mol-1; and in good agreement with the
atomistic results. The investigation gave promise towards the utility of molecular
representations of coal char within the context of fundamental coal gasification reaction
mechanism with CO2. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
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